Add waveforms

1. Add a waveform PV to your Cavity class. E/very cavity will have a PV named XXX:AFF_Set_Q
2. Use record.doubleArray() to obtain the value of this PV and store it in the Cavity class
3. Every time selection changes update plot name (implemented in #5 (closed) ) and the plot data with the value of AFF_Set_Q